Biomedical Engineering Reference
In-Depth Information
3.6
Summary and conclusions
Polymer-based composites reinforced by carbon fibers have been used for reinforcing structures.
CNTs have significantly better properties than carbon fibers making them superior fillers in compo-
sites. However, nanotubes easily agglomerate, bundle together, and entangle, thus limiting the rein-
forcing efficiency on polymer matrix. The poor dispersion along with the rope-like entanglement
leads to reduced properties of the composites. Hence, to maximize the potential of CNTs as effec-
tive reinforcements in composites, the nanotubes must be well dispersed and interact with the
matrix to prevent slippage. Dispersing nanofillers in a polymeric matrix is significantly more diffi-
cult due to their strong tendency to agglomerate. The problem is compounded if high volume frac-
tion composites are to be realized. Methods such as high-speed shearing and ultrasonication have
been attempted with limited success. To achieve strong interfacial adhesion with the surrounding
polymer matrix, the surface of the nanotubes needs to be chemically functionalized.
In addition to the traditional polymer-processing techniques (melt blending, solution casting,
and in situ polymerization) electrospinning and LbL assembly methods of compounding composites
deserve special attention. Both techniques are versatile and offer the potential to incorporate biolog-
ical entities in the composite. Electrospinning offers the potential of combining material properties
with morphological characteristics (3D) that are attractive for tissue engineering application.
Multilayered films assembled using the LbL technique appears promising for drug delivery
application.
An added advantage of using CNTs in composites is that it makes the composite conductive
which aids in bone healing and in neural stimulation. Given the importance of orientation with
respect to electrical conduction, it is important to be able to align CNTs in composites. Since higher
conductivities are desirable, it is also imperative that methods to incorporate well-dispersed nano-
tubes at higher concentration be developed.
Mixed and conflicting results have been obtained for the toxicity of CNTs. The reason lies in
the presence of impurities, degree of functionalization, types of cells used, and even the techniques
used to assess cytotoxicity. It has been suggested
[255]
that at least two or more independent tests
be used to test for toxicity of nanotubes. Results from studies conducted in vivo and in vitro are
difficult to compare. Before nanotubes can be widely accepted in medical applications, its long-
term toxicity needs to be evaluated.
References
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nanorods and nanotubes, Science 277 (1997) 1971
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[3] M. Yu, et al., Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load,
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640.
[4]
J.N. Coleman, U. Khan, Y.K. Gun'ko, Mechanical reinforcement of polymers using carbon nanotubes,
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